1. Field of the Invention
This invention relates to an air bag controlling apparatus disposed at a lower portion of a pad section of the steering wheel of a vehicle.
2. Description of the Related Art
An air bag controlling apparatus for controlling the operation of an air bag for an automobile is conventionally known wherein a deceleration of the vehicle is detected by using an acceleration sensor disposed at a suitable location of the vehicle. The acceleration sensor is integrated with respect to time to obtain a velocity component and then the velocity component is processed to render the air bag operative.
Various type of sensors are employed as the acceleration sensor, including a mechanical sensor which detects deceleration by making use of the inertia of a predetermined mass and an electric sensor which converts a strain caused by the deceleration into an electric signal using a strain gage or the like. A controller of the air bag controlling apparatus determines, based on an output signal of the acceleration sensor, whether or not the air bag should be rendered operative. In order to facilitate determination of operation of the air bag, large number of acceleration sensors must be disposed at locations where they can detect deceleration readily, such as, for example, at end portions of the vehicle.
When a large number of acceleration sensors are used as described above, the air bag controlling apparatus has a high cost cannot be mounted readily because long wiring lines are required. Therefore. investigations have been directed to reduce the number of acceleration sensors and rearrange the acceleration sensors to locations at which wiring lines to the acceleration sensors are laid readily. However, this causes the following problem.
In particular, when a deceleration signal from an acceleration sensor rises higher than a predetermined value or threshold level, if the velocity of the vehicle is low, then the air bag need not be rendered operative. Otherwise if the velocity of the vehicle is high, the air bag must be rendered operative. For example, a velocity waveform A and another velocity waveform B shown in FIG. 6, which shows time characteristics of calculated velocity values .DELTA.V each obtained by integration of a deceleration signal from an acceleration sensor, are compared with each other. The velocity waveform A indicates that the calculated velocity value .DELTA.V remains lower than a predetermined value even as time passes and accordingly, the air bag need not be rendered operative. However, at an initial stage, the velocity waveforms A and B are similar to each other. Accordingly, there is a problem in that, is difficult to judge it at an initial stage before the potential force of an imminent impact becomes excessively high, whether or not the air bag should be rendered operative. In other words it is difficult to quickly determine that the air bag should not be rendered operative with the velocity waveform A and that the air bag should be rendered operative with the velocity waveform B.
Therefore, the inventors have made a tedious investigation to solve the problem described above. As a result of the investigation, the inventors have found that the width of the waveform of the deceleration signal output from an acceleration sensor at an initial stage depends upon the velocity of the vehicle.
In particular, it has been found that, as shown in FIG. 7, the width of the waveform of the deceleration (-G) signal from an acceleration sensor at an initial stage is longer during low velocity running of the vehicle (acceleration waveform A) than during high velocity running (acceleration waveform B). The durations of the waveform widths during low velocity running and high velocity running are denoted by t.sub.L and t.sub.H, respectively. In short, it has been found that the velocity of the vehicle should be discriminated based on the duration of the waveform width.
Further, while a high degree of accuracy is required for deceleration signals from acceleration sensors as described, a sensor portion of an acceleration sensor is estimated to involve in the output thereof the dispersion of 30% or so to the highest arising from its hysteresis characteristic in the manufacturing process. Accordingly, sensitivity correction is performed for the output of each of acceleration sensors to assure the accuracy of it, which results in increase of the cost.